1. Field of the Invention
The present invention relates to an image forming apparatus that receives image data and performs print processing, an image forming method and a computer-readable medium. More particularly, the present invention relates to calibration control for stabilizing the colors of a printed image.
2. Description of the Related Art
In recent years, demand is increasing for direct imaging printers that do not require the use of plates used in offset printing or the like. Direct imaging printers are widely used to respond to the needs for reduction in print time and printing a large number of copies, as well as responding to environmental issues such as discarding of misprinted pieces of paper. Among direct imaging printers more widely used are inkjet printers, which are advantageous in terms of cost and suitable for electrophotographic printing, and electrophotographic printers, which are highly productive and produce printed items closer to offset printing quality. Under the circumstances, direct imaging printers serving as an alternative for conventional offset printing and photography are required to provide color stability for an image formed on paper, which is one of the most important functions.
In order to ensure color stability, techniques regarding color stabilization control (calibration processing) have hitherto been proposed. As an example of color stabilization control in an electrophotographic image processing apparatus, an image forming apparatus has been proposed in which an image on output paper where a patch pattern has been formed is read by a printer unit and image control is performed based on the read result. In this image forming apparatus, the apparatus for reading the patch pattern can be, for example, a reader unit connected to the printer unit, an external color/density meter, or a post-fixing spectral sensor housed in the printer unit. Also, the image forming apparatus forms a CMYK single color tone patch or a CMYK mixed color patch, as the patch pattern. The image forming apparatus generates a one-dimensional tone correction LUT or an N-dimensional color correction LUT based on the results obtained by reading such a patch.
This calibration process, however, is for entire optimization by collectively correcting predefined colors to be corrected. Accordingly, it is not possible to selectively correct only specific colors that particularly require highly accurate correction from among the colors used in a job.
To address this, Japanese Patent Laid-Open No. 2006-174240 discloses a method for selectively correcting only specific colors. According to Japanese Patent Laid-Open No. 2006-174240, first, a print job is analyzed, and all characteristic colors used in the job are listed. Then, neighboring colors to the characteristic colors are extracted from a device profile, and a chart of the extracted colors is output. The device profile is corrected based on values read from the chart.
Japanese Patent Laid-Open No. 2010-213012 discloses another correction method. Japanese Patent Laid-Open No. 2010-213012 enables a user to designate colors that he/she wants to selectively correct. If it is determined, as a result of analysis of a PDL job when printing a job, that the designated colors are used in the job, the image forming apparatus carries out calibration on only the designated colors.
The colors frequently used across a plurality of pages constituting a print job particularly require highly accurate correction. Specific examples that require highly accurate correction are objects that appear in most or all pages of a job, such as “tab marks” printed on page edges of an instruction manual or the like in order to indicate the segments of chapters, and “company logos” printed on page corners of a catalog or the like.
However, with the technique disclosed in Japanese Patent Laid-Open No. 2006-174240, all characteristic colors used in a job are listed, but no consideration is given to the use frequency across a plurality of pages, so it cannot be said that only colors that require highly accurate correction are extracted. Furthermore, only characteristic colors are extracted, and it is not possible to extract major colors composed of CMYK process colors.
Also, the technique disclosed in Japanese Patent Laid-Open No. 2010-213012 does not perform color extraction based on the analysis of a PDL job, and a print operator manually inputs information about major colors to be corrected. Accordingly, the author of data has to know detailed information about CMYK and RGB values of major colors, or the names of predefined characteristic colors. Also, in the case where the author of data and the print operator are different as is often the case in the POD market, the detailed information about major colors needs to be properly communicated.
In view of the above, the present invention provides a technique with which major colors that require highly accurate correction can be extracted more accurately and more easily.